GB2369376A - An apparatus and method for isolating the tension in a double twisted cable during twisting of the cable - Google Patents

Abstract

In apparatus for and a method of making a cable a zero twist machine 1 is connected to a double twist machine 2. A capstan 12 draws a wire pair through to the double twist machine 1 isolating the tension in the pair upstream of the capstan from that downstream of the capstan. A further capstan 17 enables the tension in the wire to be isolated from the tension in the wire at the point at which it is wound on a take up reel 18. By controlling the drives of both capstans 12 and 17, the tension in wire at entry to the double twist machine may be optimized. The provision of the two capstans enables wire to be metered into and out of the bow thus preventing of inhibiting looping of the wire out of the bow 16 of machine 2 under low tension.

Description

APPARATUS FOR AND METHOD OF PRODUCING A CABLE

The present invention relates to apparatus for and a method of producing a cable, particularly, but not exclusively a copper cored local area network (LAN) data cable.

Data cable is being installed in increasing quantities to link computers and other devices. In spite of the use of fibre optic cable for trunk data transmission lines, at the local area network level where the lines branch out to the myriad of computer terminals, copper cable is still the best option in that it is low cost, easy to install and connect, and has been developed to provide data transmission performances which match and exceed current computer requirements.

There are various standards of performance known as CATEGORY 3, CAT 4, CAT 5, CAT 5e and now CAT 6 and 7.

The cables are known as 4 pair cables, consisting of twisted'twins' or'pairs'which are simply two small solid core copper cables (overall diameter with plastic insulation-1 mm) twisted together. Four of these 'pairs'are then themselves twisted or layed up together into a bundle and sheathed in an overall plastic insulator.

The data transmission performance of such cables is measured by the following factors.

- The mount of signal which is undesirably transferred from one'pair' to another (Crosstalk, NearEnd Crosstalk, Far End Crosstalk)-This is generally optimised by ensuring that the lay length of each pair is subtly different.

- The strength of the signal or conversely ensuring that there is minimum loss of signal along a length of cable. This is affected by the resistance or in dynamic signal terms the impedance of the cable.

This is optimised by attention to the amount of stretch of the copper during processing, which work hardens the copper and increases its resistivity, and the inductive and capacitive coupling between each of the copper wires which is affected by the insulation and distance between each of the wires.

It can therefore be seen that it is the minute detail of the wire and its manufacture which assists a basically very simple product to behave as a high tech performer.

Currently there are two basic methods of production; a two stage method and a one stage method.

In the two stage method the'pairs'are made in the first stage by feeding the two wires into a double twist machine. Four reels each containing a'pair'of different lay are then fed into another single or double

twist machine to lay them up together at a relatively long lay.

The choice at the second stage between double or single twisting will depend on the quality and whether overall screening tapes are needed. The single twist is much slower, but gives the better quality and taping possibilities.

In the one stage method, two initial single wires for each pair are loaded inside the cradle of a double twislrmarhineand putted by capstan-- wheel from the inside to the outside, whereby a'pair'is created in the same way but in reverse. Four of these double twist units paying out the pairs are jointly fed from the common capstan wheel into usually a single (but could be a double) twist machine to twist the'pairs'together in a single production line. Such machines are called group twinners.

One well known way of improving the inductance of the'pairs'of wires is to try and remove any periodic induced effects due to changes in the distance between the copper conductors. It is important that the plastic insulation is concentric with the copper to hold the separation constant. In practice an eccentricity of 95% is a good result, but the actual situation is often worse, depending on the system control and tooling of the extrusion line.

Because when a'pair'is twisted in a conventional double or single

twist machine the individual wires are also twisted around themselves by one turn for each lay length, the eccentricity results in a change in the separation distance between the wires of the'air'which can lead to an increased impedance and varying impedance between pairs.

This effect can be reduced, and the quality of'pair'improved by backtwisting the individual wires before twisting them together. This has the effect of'averaging'out the eccentricity.

The degree of improvement will depend on the degree of eccentricity.

If the wires are say only 85% concentric, then the improvement will be significant.

In practice, depending on the degree of eccentricity and the type of plastic insulation used, it has been found that to backtwist by between 25% and 40% gives a good result.

It is also thought that some improvement on backtwisting is also gained because performance of the cable is affected by loss of adhesion between the copper and the plastic surface (electric conduction being largely a surface effect). As the lay length of the'pairs'can be very short (12mm) and the individual wire is experiencing a complete twist within in this short length, the copper/plastic boundary can be disturbed. The disturbance is far less if the wire is backtwisted by say 40%, at which level

the backtwist itself is only 40% as severe as the pairing twist, and the resulting/residual pairing twist is only 60% of what it would be without backtwist, keeping the wires more within the adhesion limits of the materials. Again, this particular effect may be more prevalent with plenum/PTFE fire resistant plastics which are being used more and which have poorer adhesion to copper. Backtwisting may be achieved in a variety of ways. Backtwister pay offs in which each core is twisted first in the opposite direction before being fed to a double twist machine may be used.

Alternatively, zero twist twinners have long been used for telephone cable to provide 100% backtwist cable in which there is no resultant twist in the individual wires of a'pair'.

Also, banks of'zero twist'units are traditionally put together to form what is called a'skip strander'. These are made by many suppliers, but not with rotating assembly stranders, since they are invariably used for where 100% backtwist or full planter cabling is used.

Further, zero twist twinners have long since been ganged with single twist machines in group twinners and singly to provide less than 100% backtwisted resultant cable (i. e. the 100% backtwist of the'pair'will be reduced by the twist imparted by the single twist laying up).

It has also been long since proposed to gang zero twist twinners with

double twist machines in order to maximise speed potential on these high speed bow type machines. The unfortunate effect though is that the ultimate speed is limited by the tension which accumulates, causing the wire to stretch or break at high speed.

According to one aspect of the present invention there is provided apparatus for making a cable comprising a plurality of individual wires comprising a zero twist machine for producing a pair of wires with zero twist and a double twist machine for receiving the pair output from the zero twist machine and for imparting a twist thereto, the double twist machine having a bow assembly within which a device for isolating the tension at entry to the bow assembly from that at exit from the bow assembly and a device disposed between the two machines for isolating the tension in the pair at the exit from the zero twist machine from that at the entry to the double twist machine.

According to another aspect of the present invention, there is provided a method of making a cable comprising the steps of producing a wire pair in a zero twist machine, feeding the wire pair to a double twist machine and imparting a twist therein isolating the tension in the pair at entry to the bow from that at exit from the bow and isolating the tension in the wire pair at the exit from the zero twist machine from that at the entry to the double twist machine.

In a preferred embodiment of the invention one or each device for isolating the tension comprises a driven capstan or other hauling device.

The zero twist machine comprises a bow assembly within which is disposed a pay off drum and dancer associated with the drum. A second pay off drum is disposed outside the bow assembly and a second dancer is associated with the second drum. Both dancers operate to control the tension of wire paid off from their respective drums. The two capstans or hauling devices minimise and control the tension in the bow of the double twist machine, and hold the wire pair at a fixed length between them to avoid any further possibility of the wire looping out of the bow at high speed and at lower tension. Furthermore a take up reel is also disposed in the bow assembly of the double twist machine. The capstan or other hauling device in the double twist bow assembly, in operation, isolates the tension in the cable where it enters the take up reel from that in the cable as it leaves the bow. As the tension at this latter point is at a maximum any tendency of the cable to embed itself in the turns of the cable beneath due to the high tension are reduced or eliminated.

In order that the invention may be more fully understood, one embodiment thereof will now be described by way of example with

reference to the single figure of the accompanying drawings, which shows a schematic diagram of apparatus for producing a cable.

Referring to drawing, the apparatus comprises a zero twist machine 1 and a double twist machine 2. The zero twist machine 1 comprises a bow assembly 3, a first driven pay off drum 4 disposed outside the bow assembly 3 and a second driven pay off drum 5 disposed within the bow assembly 3. Dancer rollers 6 and 7 are provided downstream of respective pay off drums 4 and 5 to provide for tension control of the associated wires 8 and 9. Pulleys 10 and 11 are provided to guide wires to and from the bow assembly 3. In operation, as the bow 3 rotates, wire 8 is wound with zero twist about wire 9. Tension in the two wires 8 and 9 is controlled automatically as the wires are paid off from driven pay off drums 4 and 5 by appropriate automatic positioning of the dancer rollers 6 and 7. On entry to the bow 3, wire 8 is guided around a guide roller 10 and on exit from the

bow the wire pair comprising the wires 8 and 9 is guided round a guide roller 11.

The wire pair 8, 9 is drawn off the guide roller 11 by means of a driven capstan 12. This capstan 12 draws the wire pair through to the double twist machine 1 isolating the tension in the wire pair upstream of the capstan from that downstream of the capstan. From the capstan 12 the

wire pair travels to a roller 13. This roller 13 is mounted on a support 14 which comprises a load cell 15 which enables the tension in the wire pair at this point to be optimized which will usually mean minimized.

Downstream of the roller 13, the wire pair enters the bow 16 and is pulled through the bow by means of a driven capstan 17 and then onto a clutch controlled take up reel 18 all disposed within the bow 16.

In any high speed twisting machine and method the major task is to minimise the tension in the wire. In a double twist machine such as machine 2 described above, the point of maximum tension is within the bow 16 at point 19. This maximum tension can reach 5KG at 5000 twists per minute on a small cable. The capstan 17 within the bow 16 enables this tension at point 19 to be isolated from the tension in the wire at the point at which it is wound on the reel 18. Without this isolation the windings on the take up reel may be too tight causing windings to embed themselves between the windings of the layer below which leads to stress/stretch in the wire during storage and to bad (jerky) unwinding when paying off into the next process.

The pulling tension at the capstan 17 inside the double twist machine 2, which is also the critical maximum tension, which will affect wire impedance, is affected by several other variables such as the frictional

effects in the bow, which increase with speed ; the effects of centrifugal force on the entrance and turn around guide pulleys and the tension at the input to the bow 16. This input tension must be kept at its lowest possible level consistent with the wire staying in the bow 16 due to the effects of windage. Bow design can minimise the effect but the point of minimum tension is about one third of the way into the bow, where the effect of the centrifugal force here tending to pull the wire into the bow, meets the frictional force trying to stop it. If the input tension is too low, the windage effect on the wire as it rotates through the air can loop it out of the bow, where the wire breaks.

As already described the capstan unit 12 isolates the input tension at the entry to the double twist machine 2 from the pulling tension of the zero twist machine 1 thus keeping this tension to a minimum. This capstan also enables the lay length in the double twist machine to be accurately fixed. This lay length is a crucial factor in the performance of the finished data cable. By controlling the drives of both capstans the tension in the wire at the entry to the double twist machine 2 may be maintained at the optimum/minimum which may be as low as 100 grams at the input to the double twist machine 2. This is even lower than a normal double twist twinning operation where typical tensions may be 1.5kg at this point. The provision of a capstan both outside and inside the double twist machine 2 enables the wire to be effectively trapped at each end of the bow being "metered"into and out of the bow thus preventing or at least inhibiting the looping of the wire out of the bow under low tension.

It will be appreciated that the above embodiment has been described by--way of -example only and that many variations are possible without departing from the scope of the invention.

Claims (12)

1. Apparatus for making a cable comprising a plurality of individual wires comprising a zero twist machine for producing a pair of wires with zero twist and a double twist machine for receiving the pair output from the zero twist machine and for imparting a twist thereto, the double twist machine having a bow assembly within which a device for isolating the tension at entry to the bow assembly from that at exit from the bow assembly and a device disposed between the two machines for isolating the tension in the pair at the exit from the zero twist machine from that at the entry to the double twist machine.

2. Apparatus for making a cable as claimed in claim 1, in which one of the devices for isolating the tension comprises a hauling device.

3. Apparatus for making a cable as claimed in claim 1 or 2, in which each of the devices for isolating the tension comprises a hauling device.

4. Apparatus for making a cable as claimed in claim 2 or 3, in which the or each hauling device comprises a driven capstan.

5. Apparatus for making a cable as claimed in claim 1,2, 3 or 4, in which the zero twist machine comprises a bow assembly within which is disposed a pay off drum and dancer associated with the

drum.

6. Apparatus for making a cable as claimed in claim 5, in which a second pay off drum is disposed outside the bow assembly and a second dancer is associated with the second pay off drum.

7. Apparatus for making a cable as claimed in any preceding claim, in which a take up reel is disposed in the bow assembly of the double twist machine.

8. Apparatus for making a cable substantially as hereinbefore described with reference to the accompanying drawing.

9. A method of making a cable comprising the steps of producing a wire pair in a zero twist machine, feeding the wire pair to a double twist machine and imparting a twist therein isolating the tension in the pair at entry to the bow from that at exit from the bow and isolating the tension in the wire pair at the exit from the zero twist machine from that at the entry to the double twist machine.

10. A method of making a cable as claimed in claim 9, in which the wires are fed from respective pay off drums and dancers operate to control the tension of wires paid off.

11. A method of making a cable as claimed in claim 9 or 10, in which the wire tension in the bow of the double twist machine is minimized and controlled and the wire pair held at a fixed length between tension isolation points to avoid any further possibility of the wire looping out of the bow at high speed and at lower tension.

12. A method of making a cable substantially as hereinbefore described with reference to the accompanying drawing.